Heavy oil is any type of crude oil that does not flow easily. Heavy oil is typically defined as crude oil having a viscosity of greater than 100 cp, or a gravity of less than 22° API. Heavy oil can be classified into three categories: 1) heavy oil having a viscosity less than 10,000 cp and a gravity between 10° and 22° API; 2) extra-heavy oil having a viscosity less than 10,000 cp and a gravity less than 10° API; and 3) bitumen having a viscosity greater than 10,000 cp, regardless of gravity. These heavy oils usually require some form of enhanced oil recovery, typically the addition of heat and or solvent to the reservoir, in order to recover the oil.
In order to recover these heavy oils, wells are usually drilled through formations that contain earth layers that may consists of different permeabilities, and/or porosities and/or saturations. These wells are often lined with casing and cement that have completions, allowing communication with these intervals, open along part of the well. Preferably, the fluid flow rate into and out of each layer (interval) should be about equal. If the permeability or water saturation of one layer (interval) is anomalously high, then fluid flow rate and/or energy flow rate in that layer is higher than in the other layers. Such a high permeability layer is called a “thief zone”, due to the energy/fluid lost in this area. These thief zones detrimentally affect the production rate from the recovery of heavy oils by stealing the energy added to heat the heavy oil
One of the best ways to seal the thief zones involves in-situ sealing deep inside the formation. Silva et al.'s mathematical model (“Waterflood Performance in the Presence of Stratification and Formation Plugging,” SPE Paper No. 3556, 46th SPE Annual Meeting, New Orleans, Oct. 3 through 6, 1971) showed that sealing a thief zone at the wellbore was not enough. When the thief zone is sealed only near the wellbore, the flow behavior continued just beyond the sealed region as if no sealing had occurred. Unfortunately, most known methods tend to seal the thief zones only near the wellbore.
Another commonly utilized technique is to cause an in-situ change of the injection fluid in order to deeply penetrate the thief zone before sealing takes place. Two such commercial methods are now being used. One method is in-situ polymerization of carefully spaced slugs of monomer and catalyst. In theory, mixing these slugs deep inside the formation produces the desired polymer matrix and seals the thief zone. However, given the complexity of flow behaviors of slugs in a heterogeneous formation, it is difficult to place the polymer matrix where desired. Furthermore, if the slugs are not properly placed it is difficult to ensure that the entire thief zone is sealed off. The other method is time-delayed gelling of polymers. But the delay time may only be a few days (e.g., 48 to 72 hours for xanthan gum by chromium ions). Such short times prevent a deep penetration of the sealing fluid into the thief zone.
Techniques have also been designed to capitalize upon the heat added to enhance the oil recovery. U.S. Pat. No. 3,669,188 teaches using a plugging fluid that reacts to deposit a plugging material as temperature increases. The thief zone near the well is heated so that it is hotter than the surrounding regions, then the plugging fluid is injected into that zone, and then unreacted plugging fluid is displaced after plugging has occurred. Preferably, the plugging fluid is an aqueous solution of a metal and a reactant. The metal precipitates as a gelatinous metal hydroxide and the reactant increases the solution pH to cause that precipitation. Preferably, slugs of hot water are used to heat the thief zone. This injected hot water, however, may lose a substantial amount of energy, making the process less effective deep inside the thief zone, where plugging is needed the most. Basically, this method was designed to apply in the “near well” region (less than twenty feet from the wellbore).
Other techniques have been designed to incorporate secondary heating sources utilized specifically to heat plugging material in the thief zone. U.S. Pat. No. 3,620,302 teaches plugging thief zones with an inorganic silicate. An aqueous solution of the silicate is injected into the thief zones, in-situ combustion is generated in a nearby zone, and that combustion is sustained until enough heat is transferred to the thief zones to cause the silicate to intumesce and seal the thief zones. Unfortunately, such in-situ combustion cannot be used in most fields.
To combat the problems associated with in-situ sealing of the thief zone, techniques have been designed to incorporate electrical currents to seal the thief zones. U.S. Pat. No. 4,809,780 teaches a method of plugging the thief zone that involves in-situ transformation of the thief zone. The in-situ transformation is performed by inducing electrical resistance through a saline solution that is injected through the injection well. However use of in-situ electrical transformation of the saline solution to seal thief zones cannot be utilized for tar sands because the electrical current is confined to the narrow path of least resistance and accordingly cannot completely seal the thief zone.
There exists a need for a method able to restrict heat loss over large areas of thief zones that is applicable over different types of strata.